1. Field of the Invention
This invention is directed generally to an apparatus and method for growing large single crystals of metal halides and chalcogenides and more particularly to the size-scale-up technology in the growth and preparation of single-crystal optical components (such as windows and lenses) of metal halides and chalcogenides.
2. Prior Art
The common procedure for preparing IR windows from metal halides or chalcogenides is to grow circular cylindrical ingots of a diameter exceeding that of the window to be fabricated, and of a cylindrical length (after the conical ends have been cut off) several times the thickness of the window. With this shaping of ingot, it is not practical to grow an ingot that will yield one window, for the conversion ratio of usable to discarded material would be very poor. These prior art IR window production methods therefore require several steps of fabrication, viz cutting, grinding, and polishing. As a rule, as the number of fabrication steps required to convert the ingot to the finished product increases, the conversion ratio of the material decreases. This phenomenon has the effect of increasing the cost of the finished product.
Other disadvantages of the prior art methods are contamination problems, associated with the multiple fabrication steps, and the attendant technical and economic difficulties in scaling up the crystal growth process to yield IR windows having large diameters.
The difficulty in producing large IR windows or other large optical components by conventional crystal growth processes may be defined as a production scale-up problem. Since conventional crystal growth processes yield optical components at a relatively low conversion ratio, the amount of material required to yield large IR windows is exceedingly large. For an increase in the linear dimension of an optical component by a factor of X=6, there will be an associated increase in the mass of material required to yield the components by a factor of 216. A scaled-up boule of potassium chloride of a diameter of 12 inches will weigh more than 150 kg. Obviously, the production and handling of such a mass will require special machinery occupying precious space and a scaled-up more elaborately instrumentalized furnace necessitating a high power input.
These observations clearly point to the necessity of casting the single-crystal ingot during its growth to a shape which, while amenable to the constraints of growth of materials of high crystalline quality, will require the minimum of fabrication to the configuration and finish of the final product. Applicant knows of no prior art crystal growth process which meets the above stated requirements.